The present invention relates to a shut-off valve and, in particular, to an improved seal design used for such valves of the piston type.
A valve housing is formed with a flow channel, or flow duct, which communicates an inlet with an outlet to permit the flow of a medium therethrough. A shut-off valve utilizes a closing-off element which is selectively movable in the housing to block the flow duct. In a piston type of shut-off valve, the closing-off element is an axially displaceable piston which is sealed in the housing by at least one sealing ring made of a compressible material.
The sealing ring with which the piston is operatively associated to block flow is subject to heavy wear. The sealing ring can, for example, be seated at the periphery of the flow channel, and the piston is inserted into the sealing ring to seal the flow channel. When the piston is drawn out of the sealing ring thereby opening up the flow channel, the sealing ring is exposed continuously to the flow of the medium. This inherently imposes a heavy load of wear on the sealing ring, given the possible flow speeds, and the load is still further increased in piston positions just slightly spaced from the sealing ring because then even higher flow speeds occur. Furthermore, a deflection in the flow channel typically takes place in the vicinity of the seal because the flow channel is not straight Therefore, the flow encounters the inner cylindrical face of the sealing ring not only at a tangent, but also produces a radially directed component. Aside from this, the sealing ring is radially compressed each time the piston plunges inward, because only in this way is sealing possible. Accordingly, upon each closing movement, a squeezing effect takes place, followed by a sliding motion for the compressed part of the seal, associated with friction, and the sliding motion also takes place during the movement for opening the flow channel. The flow strains, squeezing strains and friction strains cannot long be withstood by conventional sealing rings, especially at elevated temperatures. The sealing ring can also be carried on the piston to engage the inner wall of the flow channel. Such an arrangement also subjects the sealing ring to much the same type of wear as discussed just above.
Expanded graphite, such as is available from Union Carbide Corporation under the brand name Grafoil, has the desirable characteristic of being essentially independent of temperature as far as its operating characteristics are concerned. However, it has the drawback of not being sufficiently resistant to erosion when exposed to a flowing medium. Prior art sealing rings that are made of expanded graphite are produced by compression molding of a winding that is coaxial with the piston axis and formed from a sheet of graphite of approximately the width of the axial dimension of the sealing ring. However, valves equipped with this type of sealing ring have a relatively short service life because the erosion forces generated by the flowing medium are applied to the planar surface area of the sheet of graphite which is not sufficiently resistant to erosion and, moreover, because known seals of this kind are not sufficiently dense. When seal erosion occurs to produce a leak, re-tightening by axially compressing the sealing ring is not a good solution because it causes uncontrollable deflection of the various layers of the winding, resulting in non-homogeneities that have a deleterious effect on the service life. Moreover, this winding type of sealing ring tends to wrinkle when it is axially compressed without an abutment at its inner and outer periphery. When such an abutment is present, as is the case with a valve, wrinkling cannot occur but the same forces that cause wrinkling in the absence of an abutment produce areas with a localized, variable, and highly elevated pressure in the presence of an abutment. Consequently, the force of actuation increases with each re-tightening, because the innermost layer of the winding abuts against the piston with a flat surface and, in particular, with variable, locally highly elevated pressure areas. When it is subjected to an oncoming flow, particles become easily detached from it.